Distributed propulsion
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In aeronautics, Distributed propulsion is an arrangement in which the propulsive and related air flows are distributed over the aerodynamic surfaces of an aircraft. The purpose is to improve the craft's aerodynamic, propulsive and/or structural efficiency over an equivalent conventional design. Anticipated benefits include improved
fuel efficiency Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, wh ...
, emissions, noise, landing field length and handling. Distributed propulsion may be accomplished by spanwise distribution of partially or fully embedded multiple small engines or fans along the wing. Alternatively, it may involve ducting exhaust gases along the wing's entire trailing edge.


Design principles


Definition

Distributed propulsion on an aircraft is typically characterised not only by the distributed nature of the propulsive thrust but also by utilisation of the effect this has on the aircraft aerodynamics.Epstein, A. H. (2007
"Distributed Propulsion: New Opportunities For An Old Concept"
MIT. (retrieved 16 June 2022).
The propulsive air flows are distributed over the aerodynamic surfaces of the craft, typically spanwise over a fixed wing. These flows may interact with other air flowing over the wing and substantially affect the
aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
. However there is no accepted formal definition.Kim, Hyun Dae. (2010
"Distributed Propulsion Vehicles"
27th International Congress of the Aeronautical Sciences, ICAS 2010, pp. 1–11. (retrieved 16 June 2022)
Burston et al
"Design principles and digital control of advanced distributed propulsion systems"
in: Karakoç et al (ed). ''Energy'' special issue on ''Emerging Energy Technologies and Alternative Fuels for Aviation'', Volume 241, 15 February 2022.
Three broad classes of distributed propulsion system have been identified: *Distributed exhaust, such as jet flaps. *Multiple discrete propulsors (fans, propellers or jets), which may be powered individually or by remote drive from fewer engines. *
Cross-flow fan Crossflow may refer to: * Ford Crossflow, the 1967 version of the Ford Kent engine * Crossflow cylinder head * Cross-flow fan, a type of mechanical fan * Cross-flow filtration, a filtration technique * Cross-flow turbine A cross-flow turbine, B ...
s, which are a type of horizontal-axis rotor.


Aerodynamic functions

In addition to providing propulsion, distributed propulsion arrangements have been studied with a view to providing various aerodynamic functions. These include: * Direct reenergizing of the boundary layer * Flow separation control * Powered lift/circulation control * Viscous drag reduction * Vortex/vorticity control * Vehicle control/vectored thrust


Potential benefits

Several areas have been identified in which distributed propulsion may offer benefits over conventional designs. These include fuel efficiency, noise abatement, steep climbing for short take off and landing (STOL), novel control approaches (in particular eliminating control surfaces for roll, pitch and yaw moments), and high bypass ratios. It has also been suggested that smaller propulsors will be cheaper to manufacture and easier to handle during assembly and maintenance.


Distributed propulsors

The multiple propulsion unit strategy involves three or more propulsion units. These units are arranged in Leader or Follower configurations. They are classified into five intensity classes (A–E) and three thrust-to-weight ratio categories (I-III). They can be arranged within/above/around or across the wing(s)/fuselage(s) or airframe. Leader arrangements employ propulsion units to directly generate thrust, i.e., distributed engines. The Follower arrangement uses secondary propulsion unit(s), such as multiple fans that are powered by a single engine. In the last case, the power transmission between the fans and engines may be linked by ducting hot gas, mechanical gears, or electric power lines.


Distributed electric propulsion

Distributed electric propulsion (DEP) comprises multiple small fans or propellers driven by electric motors. Typically, each individual thruster is direct driven by its own relatively small and lightweight electric motor. The electrical power may be provided by any suitable source. The advantages of distributed propulsion for lightweight, high aspect ratio solar-powered aeroplanes are exemplified in the AeroVironment HALSOL/Pathfinder/
Helios In ancient Greek religion and Greek mythology, mythology, Helios (; grc, , , Sun; Homeric Greek: ) is the deity, god and personification of the Sun (Solar deity). His name is also Latinized as Helius, and he is often given the epithets Hyper ...
projects, begun in 1983, and the
University of Michigan X-HALE A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, th ...
, flown from around 2012. Distributing the electric motors along the span was able to control how the airframe flexed in flight, allowing the structure to be much lighter than the conventional rigid equivalent.


Aeroelasticity

When heavy propulsion units are distributed along a wing, this allows the wing structure to be made lighter. However their weight and thrust can interact with the natural tendency of the wing to flex under varying loads (
aeroelasticity Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classif ...
). This can cause problems, for example it was a major cause of a crash involving the
NASA Helios The Helios Prototype was the fourth and final aircraft developed as part of an evolutionary series of Solar energy, solar- and fuel cell, fuel-cell-system-powered unmanned aerial vehicles. AeroVironment, AeroVironment, Inc. developed the vehicles ...
research aircraft. One solution investigated is the use of active aeroelastic controls to correct or even make use of wing flexing during flight.Nhan T. Nguyen, Nhan T. et. al. (2018
"Distributed Propulsion Aircraft with Aeroelastic Wing Shaping Control for Improved Aerodynamic Efficiency"
NASA. (retrieved 26 June 2022)


History

Multi-engine installations have been a feature of aeroplanes since the introduction of the
Sikorsky Ilya Muromets The Sikorsky ''Ilya Muromets'' (russian: Сикорский Илья Муромец) (Sikorsky S-22, S-23, S-24, S-25, S-26 and S-27) were a class of Russian pre-World War I large four-engine commercial airliners and military heavy bombers used ...
shortly before World War One. However most do not significantly modify the airflow over the wings and are not always treated as distributed propulsion. In 1963 the
Hunting H.126 The Hunting H.126 was an experimental aircraft designed and built by British aviation company Hunting Aircraft. The aircraft was developed in order to test the performance of blown flaps, which were commonly known in Britain as "jet flaps", At ...
research aircraft was built to investigate the direct use of a jet flap for propulsion, while the
ShinMaywa US-2 The ShinMaywa US-2 is a large Japanese short takeoff and landing amphibious aircraft developed and manufactured by seaplane specialist ShinMaywa (formerly ''Shin Meiwa''). It was developed from the earlier Shin Meiwa US-1A seaplane, which was ...
flying boat of 2003 used blown flaps to improve short takeoff and landing (STOL) performance and subsequently entered production.
FanWing The FanWing is an aircraft configuration in which a horizontal-axis cross-flow fan is used in close conjunction with a fixed wing. The fan forces airflow over the fixed surface to provide both lift and forward thrust. The concept was initially dev ...
began development of the crossflow fan as a combined lift and propulsion system in 1997 and over the next few years flew several models and research drones. Subsequent research in the US focused on the use of a crossflow fan inset into the wing upper trailing edge, as the primary driver for boundary layer control and jet flap propulsion. More recently, several
unmanned aerial vehicle An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft without any human pilot, crew, or passengers on board. UAVs are a component of an unmanned aircraft system (UAS), which includes adding a ground-based controller ...
(UAV) projects have explored the potential of distributed propulsion to offer noise abatement, fuel efficiency and short-field performance. As of 2022 a manned
X-plane The X-planes are a series of experimental United States aircraft and rockets, used to test and evaluate new technologies and aerodynamic concepts. They have an X designator within the US system of aircraft designations, which denotes the exper ...
, the
X-57 Maxwell The NASA X-57 Maxwell is an experimental aircraft being developed by NASA, intended to demonstrate technology to reduce fuel use, emissions, and noise. The first flight of the X-57 is scheduled to take place in 2023. Development The experiment ...
is under development at NASA and several prototypes of a light aircraft, the
Lilium Jet The Lilium Jet is a prototype German EVTOL, electric vertical take-off and landing (eVTOL) electrically powered airplane designed by Lilium GmbH. A seven-seat production version is planned. History Initial design studies included forward-foldi ...
, have flown in Germany.


List of aircraft with distributed propulsion

*
Aurora XV-24 LightningStrike The Aurora XV-24 LightningStrike is an experimental unmanned aerial vehicle created by Aurora Flight Sciences and partners Rolls-Royce and Honeywell. It was developed for the Vertical Take-Off and Landing Experimental Aircraft program. Developm ...
: Distributed electric fans. Research UAV. Flew in 2016. *
FanWing The FanWing is an aircraft configuration in which a horizontal-axis cross-flow fan is used in close conjunction with a fixed wing. The fan forces airflow over the fixed surface to provide both lift and forward thrust. The concept was initially dev ...
: Cross-flow fan. Series of research UAVs. *
Hunting H.126 The Hunting H.126 was an experimental aircraft designed and built by British aviation company Hunting Aircraft. The aircraft was developed in order to test the performance of blown flaps, which were commonly known in Britain as "jet flaps", At ...
: Jet-flap. Manned research aircraft. Flew from 1963. *
Lilium Jet The Lilium Jet is a prototype German EVTOL, electric vertical take-off and landing (eVTOL) electrically powered airplane designed by Lilium GmbH. A seven-seat production version is planned. History Initial design studies included forward-foldi ...
: Distributed electric fans. Series of manned prototypes. *
NASA X-57 Maxwell The NASA X-57 Maxwell is an experimental aircraft being developed by NASA, intended to demonstrate technology to reduce fuel use, emissions, and noise. The first flight of the X-57 is scheduled to take place in 2023. Development The experiment ...
(Sceptor): Distributed electric fans. Manned research aircraft. Under development.


References

{{Reflist, 30em Aircraft propulsion components